1. Field of the Invention
The present invention relates to a double-tuning circuit of a television tuner, the double-tuning circuit including a trap circuit that is formed therein to attenuate an image frequency component of a television signal.
2. Description of the Related Art
In general, a double-tuning circuit of a television tuner is provided between a radio frequency (RF) amplifier circuit for amplifying a received television signal and a frequency mixing circuit for converting a television signal into an intermediate-frequency signal so as to connect the RF amplifier circuit and the frequency mixing circuit. Such a double-tuning circuit usually includes a primary tuning circuit in which a first inductor is connected in parallel with a first variable capacitive element and a secondary tuning circuit in which a second inductor is connected in parallel with a second variable capacitive element, and the first and the second inductors are inductively coupled. Some double-tuning circuits of a television tuner of this type include an image trap circuit that is formed therein and that attenuates an image frequency component of a television signal. Many methods have been proposed for forming such an image trap circuit, and one of them is a method employing an adjustable copper-foil pattern (see Japanese Registered Utility Model No. 3108713).
FIG. 9 is a diagram showing a structure of a double-tuning circuit of a television tuner disclosed in Japanese Registered Utility Model No. 3108713.
The double-tuning circuit of the television tuner shown in FIG. 9 includes a first inductor 1, a second inductor 2, a first variable capacitive element 3, a second variable capacitive element 4, direct-current (DC) blocking capacitors 5 and 6, a first coupling capacitor 7, two second coupling capacitors 8(1) and 8(2) that are serially connected, buffer resistors 9 and 10, a copper-foil pattern 11, a hot-side input terminal 12H, a cold-side input terminal 12C, a hot-side output terminal 13H, a cold-side output terminal 13C, and a tuning voltage application terminal 14. In this case, the two second coupling capacitors 8(1) and 8(2) that are serially connected are selected in such a manner that the total capacitance thereof is identical to the capacitance of a second coupling capacitor of this type whose capacitance is known.
The first inductor 1 and the first variable capacitive element 3 connected in parallel constitute the primary tuning circuit, and the second inductor 2 and the second variable capacitive element 4 connected in parallel constitute the secondary tuning circuit. One terminal of a circuit portion constituted by the two second coupling capacitors 8(1) and 8(2) that are serially connected is connected to the hot-side output terminal 13H, and the other terminal thereof is connected to a hot-side input terminal 16H. A fixed part of the copper-foil pattern 11 is connected to a connection point at which the two second coupling capacitors 8(1) and 8(2) are connected to each other, and a tip part 11a of the copper-foil pattern 11, that is a free end thereof, extends to near the first inductor 1.
Moreover, an upstream circuit of the double-tuning circuit of the television tuner includes a hot-side output terminal 15H, a cold-side output terminal 15C, a radio frequency amplifier (RF AMP) 17, an antenna input terminal 18, an inductor 19, feedback resistors 20 and 21, a bypass capacitor 22, and a power-source terminal 23. Furthermore, a downstream circuit of the double-tuning circuit of the television tuner includes the hot-side input terminal 16H, a cold-side input terminal 16C, a frequency mixing circuit (MIX) 24, a local oscillator (L OSC) 25, and an intermediate-frequency signal output terminal 26.
In the double-tuning circuit of the television tuner having the above-described structure, a trap circuit for attenuating an image frequency component in an RF signal is formed by coupling the first inductor 1 and the tip part 11a of the copper-foil pattern 11, and the capacitance of the trap circuit is an infinitesimal capacitance generated by coupling the first inductor 1 and the tip part 11a of the copper-foil pattern 11. This trap circuit can attenuate an image frequency component in an RF signal. The image frequency component to be attenuated by the trap circuit can be adjusted by changing the capacitance ratio between the two second coupling capacitors 8(1) and 8(2) without changing the total capacitance of the two second coupling capacitors 8(1) and 8(2).
In an example of the related art described in Japanese Registered Utility Model No. 3108713, the image frequency component to be attenuated by the trap circuit can be adjusted by changing the capacitance ratio between the two second coupling capacitors 8(1) and 8(2). Moreover, a trap frequency can be adjusted by adjusting the infinitesimal capacitance generated by coupling the first inductor 1 and the tip part 11a of the copper-foil pattern 11 or by adjusting a degree of coupling between the first inductor 1 and the second inductor 2 (the distance between the inductors).
However, even if a trap frequency is adjusted to the image frequency of a specific channel by adjusting the capacitance ratio between the two second coupling capacitors 8(1) and 8(2), the infinitesimal capacitance generated using the copper-foil pattern 11, or the distance between the inductors, the trap frequency does not match the image frequency of another channel. Thus, resistance to image degradation cannot be uniformly achieved between channels.
The present invention has been made in light thereof, and the present invention provides a double-tuning circuit of a television tuner capable of realizing resistance to image degradation uniformly between channels over a wide band.